To determine the position of a mobile unit equipped with a GPS receiver, measurements of distance between the GPS receiver and GPS satellites are made at the receiver. In addition to the distance measurements, the time at which the GPS signals are transmitted from GPS satellites should be deduced from the distance measurements for correcting timing errors in the receiver.
Timing errors may be effectively solved by differential GPS (DGPS) technology, in which the correction information of the timing errors is provided from a stationary reference receiver, which uses its known position to calculate timing. This is a result of the facts that, if two receivers are fairly close to each other (compared to the satellites far away in the space), the satellite signals that reach both of the receivers will have traveled through virtually the same slice of atmosphere, and so will have virtually the same errors. There are numerous satellites “in sight” (GPS satellite constellation is designed such that four or more satellites can be seen simultaneously at any location on the surface of the earth), and the reference receiver has no way of knowing which of the many available satellites a moving receiver might be using to calculate its position, thus the reference receiver runs through all the visible satellites and computes each of their errors. Then it transmits the correction information to the moving receiver. The moving receivers get the complete list of errors and apply the corrections for the satellites they're using.
There are, however, several problems in current geolocation technologies. The power consumption at a GPS receiver is always one of the major concerns in view of the portability of the mobile unit. The more data processed at the receiver, the more profound the problem. Having a GPS receiver receive plural signals and then calculate its position requires extensive processing power.
Effective detection of the GPS signals is another issue, since there are numerous satellite signals in space. Furthermore, sometimes there are not enough satellite signals received at the receiver for position calculation purposes, typically when the signals from one or more satellites are blocked from reaching the receiver. This may happen, for example, when the mobile unit is in a subway or in a building. The present invention helps to solve or minimize the above problems.